Initial Fuchsia support.

This change adds Fuchsia as a valid OS. Future changes
will add proper toolchain support.

Bug: 119831161
Test: Compile walleye. Confirm that adding `fuchsia` to the
os-specific declaration of an arbitrary target does not throw
a build error.
Change-Id: I64eb928afb7512f3fbe32abb313b4c3efe16b169

android/config.go

@@ -586,6 +586,10 @@ func (c *config) UnbundledBuildPrebuiltSdks() bool {
 	return Bool(c.productVariables.Unbundled_build) && !Bool(c.productVariables.Unbundled_build_sdks_from_source)
 }
 
+func (c *config) Fuchsia() bool {
+	return Bool(c.productVariables.Fuchsia)
+}
+
 func (c *config) IsPdkBuild() bool {
 	return Bool(c.productVariables.Pdk)
 }